1. Field of the Invention
The invention is generally directed to the field of vision correction by laser photoablation of corneal tissue, and more particularly, to a method and system for evaluating the safety of a prospective secondary LASIK treatment.
2. Description of Related Art
There are many approaches for correcting or improving a person's less than perfect vision. These include, for example, corrective spectacles, contact lenses, implanted lenses, and various forms of surgery on the eye. Over the past several years, tens of thousands of patients have undergone a type of refractive surgery known as laser in situ keratomileusis, commonly referred to as LASIK. In a typical LASIK procedure, the patient's vision defects are measured (typically limited to focussing and astigmatism errors), a hinged flap of corneal tissue is opened to expose a stromal layer of the cornea, and a laser beam is scanned over the exposed stromal tissue to ablate selected regions of the cornea. The corneal flap is then closed, and the cornea's new shape should provide the desired vision correction for the patient.
For a variety of reasons, an initial LASIK procedure does not always accomplish the desired vision correction goals in all patients. This can be due, for example, to eye movement during the surgery resulting in a decentered ablation, misalignment of the eye during surgery, inducement of secondary vision defects from the vision correction procedure itself, and for other reasons. It may be possible, however, to further correct the patient's vision with a secondary LASIK procedure. One of the primary considerations of a secondary LASIK procedure (and also the primary LASIK procedure) is whether the patient's cornea is thick enough to support the removal of tissue necessary to effect the corrective corneal profile. Based upon typical standards of care, the minimum residual corneal thickness of a person's eye should be greater than about 250 microns.
Corneal thickness is currently measured by measuring the anterior and posterior corneal position or corneal topography. This technique, however, is dependent on the ability to detect the edges or interfaces of corneal surfaces to determine their exact locations. While these techniques work well on a cornea that has not had refractive surgery such as LASIK, it is difficult to determine the position of the posterior corneal surface under the keratectomy, or LASIK flap, that is part of the LASIK procedure. The lack of precision in detecting the posterior corneal surface results in a poor estimate of post-LASIK corneal thickness. An accurate measurement of the corneal thickness remaining after a primary LASIK treatment is necessary to evaluate the safety of any secondary LASIK treatment and to determine the treatment plan.
The inventor has recognized the need for accurately measuring the post-LASIK corneal thickness of the patient's eye to evaluate the safety of a prospective secondary LASIK treatment. Accordingly, a system and method are presented below for making the necessary evaluation.